Were the Seahawks’ and Broncos’ ticket bans legal?

FORTUNE — Over the weekend, the National Football League’s Seattle Seahawks beat the New Orleans Saints, and the Denver Broncos beat the San Diego Chargers. By Sunday night, both teams were saying: “No tickets for you!” to fans of their next opponents.

The Seahawks and Broncos each announced on their websites that they would sell tickets to the NFC and AFC Championship games, respectively, only to fans from specific states. “Tickets for the NFC Championship Game to be played on Sunday, January 19 at CenturyLink Field will go on sale Monday, January 13 at 10 a.m,” stated an article posted Saturday night on the official Seahawks website. “Tickets will be available to fans with a billing address in WA, OR, MT, ID, AK, HI and the Canadian provinces of British Columbia and Alberta.”

The Broncos followed suit on Sunday night after their win, as the Denver Post noted on its blog: “Tickets [to the AFC Championship Game] will be available only to those with a billing address in the Rocky Mountain states, including Colorado, New Mexico, Utah, Wyoming, Nebraska, Montana, South Dakota and Western Kansas.”

It’s no surprise that the Seahawks’ list of lucky states, mostly in the Pacific Northwest, is missing California, home to their opponent, the San Francisco 49ers. Similarly, you won’t find Massachusetts, Maine, Vermont, or any states in the core territory of the New England Patriots on the Broncos’ approved addresses.

This is understandable. Each team wants to cater to its own fans first. But make no mistake: They were happy to keep out opposing fans in the process. It turns out that one of the benefits of having home-field advantage — enjoying an unbroken sea of supporters roaring at your back — is too good to give up. (On Monday, 49ers coach Jim Harbaugh grudgingly gave the Seahawks credit: “I actually respect it, what you’re trying to do for your team, put them in the best possible position to win that you can,” he told the press.)

So were these moves legal? The answer isn’t so simple. It is not explicitly illegal to sell only to specific states. But according to Jon King, a sports lawyer with Hagens Berman Sobol Shapiro in San Francisco (and an admitted Niners fan), fans in the barred states could have grounds to bring legal action against the teams and Ticketmaster, which administered the ticket sales but didn’t set the policy.

Other attorneys interviewed by Fortune said the same. “Public accommodation” law, argues Roger Abrams, a law professor at Northeastern University in Boston, might offer one basis for a suit. As he puts it: “Public accommodation … refers to a public enterprise that opens its doors to the public but is precluding others from ‘trading,’ that is, spending their money to buy a ticket.”

A stronger claim, in California, could be based on that state’s Unfair Competition law, which King says is “extremely broad, and one of the most consumer-friendly laws in the nation.” It allows a claim to be brought for any “unlawful or unfair business practice.” It isn’t difficult to imagine a fan arguing that the restricted sale is unfair business. And King says, “it is almost as murky as it sounds; courts have a lot of latitude to go into what’s fair and not.”

Of course, there is little time to rush to court and obtain an injunction this week before the game. But as a matter of principle, you still might see a lawsuit. It’s important to note that although this practice is rare, this is not the first time NFL teams have done it. In 2001, the New York Giants did the same to Philadelphia Eagles fans for the NFC semifinal game. In 2008, the Tampa Bay Buccaneers locked out Giants fans for a playoffs wildcard game. But neither of those examples was as significant a game as the conference championship.

If this trend continues, expect legal action down the road. “I expect this will be a topic of discussion at future team owner meetings,” King says. An NFL spokesperson says the league has nothing to do with the ticket sales: “There is no NFL policy. Teams may sell tickets to their fans in any manner they see fit.” Individual teams handle all sales of tickets to their own games, including those in the playoffs. The only exception is the Super Bowl, which the NFL handles.

For now, Abrams says that “What they have done is probably legal, albeit very stupid and certainly contrary to the spirit of the NFL. It is an issue in which the NFL should get off its butt and say something. They couldn’t ban people because they were black or Latino — the NFL would have to step up then. And sports fans bring suits for sillier reasons than this.”

The Broncos, for their part, stand by their right to sell to whomever they like. “We had an extremely limited inventory of tickets remaining for postseason games,” the team said in a statement sent to Fortune. “We wanted to ensure our local fans had an opportunity to purchase tickets by implementing these procedures.” The nearly 3,000 tickets the team made available Monday morning sold out, the team says, in about 10 minutes. (The Seahawks did not respond to requests for comment.)

Fans based outside the teams’ authorized areas for buying tickets can still do so on the secondary ticketing market on resale sites like StubHub and NFL Ticket Exchange, which is owned by Ticketmaster. And when other events such as concerts have sold in similar ways on Ticketmaster in the past, the reasoning provided is often that it is an effort to reduce the number of out-of-state scalpers who buy tickets with no intention of going. (Ticketmaster had no comment for this story.)

Whatever the reason, many fans aren’t pleased and have taken to social media to express their frustrations. As @RyanTomsic_BCM tweeted, “Seahawks are weak and scared for limiting tickets sales to certain states. How is that fair or legal. NFL cannot allow this.” As for the NFL’s insistence that it’s not a league issue, King says, “There is something inherently troubling about economic discrimination based on where someone is from. I’m not impressed by the NFL’s ‘any way they see fit’ statement.”

Finally, exclusionary policies such as these ignore the many fans who root for teams from other states. There are certainly Seahawks fans living in California. But they will find it hard to participate as the team’s “twelfth man” on Sunday.